1. Field of the Invention
The present invention relates to an apparatus for operation on a cornea using laser-beam to correct a refraction error of an eye by excising a surface of the cornea of an eye. More particularly, the present invention relates to an ablation apparatus for ablating an object by laser beam (typically an excimer laser) having a non-uniform beam intensity of Gaussian distribution in one direction and a uniform beam intensity in the vertical direction, and an apparatus for correcting the myopic astigmatism of an eye.
2. Description of Related Art
Recently, a method has been executed for correcting the refraction of an eye by ablating the surface of the cornea to change the curvature of the cornea, e.g. Photorefractive Keratectomy (PRK). In this method, it is necessary to control the depth of the ablation area so that it is uniform. Japanese Patent Application No. HEI 2-416767 (U.S. application Ser. No.812,819), the title is ABLATION APPARATUS FOR ABLATING AN OBJECT BY LASER-BEAM, filed by this applicant proposes the method for ablating an object in uniform depth by translational scanning the laser beam having a uniform beam intensity in one direction and a non-uniform beam intensity of Gaussian distribution in the vertical direction, e.g., an excimer laser beam, into the non-uniform beam intensity distribution direction.
Another method has also been proposed for correcting the refraction error of an eye. Specifically, in this method, the myopia is corrected by excising a cornea so that an ablation area forms a convex lens appearance as shown in FIG. 1 by gradually changing a diameter of a circular variable diaphragm so as to extend its size from small to larger, or to reduce from large to smaller. The astigmatism is corrected by excising the cornea so as to form a cylindrical lens appearance as shown in FIG. 2 by gradually changing a width of a variable slit the same as above.
However, there are some problems with the methods mentioned above. In the former method employing a laser beam which has a uniform beam intensity in one direction and a non-uniform beam intensity of Gaussian distribution in the vertical direction, the ablated area of the cornea can not have a uniform depth unless the laser beam has an almost uniform beam intensity in one direction. Due to the insufficiency of the alignment of the laser beam resonator, the uniformity of the laser beam emitted from a laser source is different between each laser oscillator. When beam intensity distribution in a direction to be uniform is not uniform so that it cannot be disregarded as reference to FIG. 3, and although the depth of ablation in a scanning direction can be uniform, a non-uniform beam intensity distribution remains in the vertical direction, therefore, a uniform depth in a whole ablation area can not be obtained.
Additionally, there is a similar problem as that mentioned above when beam intensity on a surface of an object is non-uniform due to problems in the optical system for transferring the laser beam into an object to be ablated.
In the latter method of changing a diameter of a variable diaphragm or variable slit, it is necessary to confine the irradiation area by employing a circular variable diaphragm when correcting the myopia, or by employing a variable slit when correcting the astigmatism. Therefore, the structure is complex and its operation becomes complicated. When correcting myopic astigmatism, it is necessary to repeat operations for correcting with different diaphragms, that is, correcting the astigmatism by using a variable slit after correcting the myopia by using a circular variable diaphragm, or in reverse order.